First of all, not all programs allow retroactive of posting FF miles. Some programs only allows own flights but not partner flights. You would need to research this first.

Assuming it can be post retroactively, then which program to post would depend on what would be your travel pattern in the future and how would you use your miles, such as international long haul flights or domestic short haul flights.

You would be able to make a better, educated decision, after learning the nuance of both the AAdvantage program and the BAEC program (Avios) - they work quite differently and each has its pros and cons. They almost are at the opposite spectrum when it comes to meet one's travel needs.